1. Field of the Invention
The present invention involves mono- or multi-color printheads and particularly heater plates for four color roofshooter printheads. The present invention also involves the use of switching circuitry for controlling the actuation of a plurality of heating elements in a mono- or multi-colored thermal ink jet printhead.
2. Description of Related Art
There are two general configurations for thermal drop-on-demand inkjet printheads. In one configuration, droplets are propelled from nozzles in a direction parallel to the flow of ink in ink channels and parallel to the surface of the bubble generating heating elements of the printhead, such as, for example, the printhead configuration disclosed in U.S. Pat. No. 4,601,777 to Hawkins et al. This configuration is sometimes referred to as "edge or side shooters". The other thermal ink jet configuration propels droplets from nozzles in a direction normal to the surface of the bubble generating heating elements, such as, for example, the printhead disclosed in U.S. Pat. No. 4,568,953 to Aoki et al. and U.S. Pat. No. 4,789,425 to Drake et al. This latter configuration is sometimes referred to as a "roofshooter".
In roofshooters, it is often desirable to supply ink to the nozzles via a passageway through the heater plates. This is the most advantageous choice because the proximity of the paper to the printhead makes any other design approach difficult. In a commercial drop-on-demand thermal inkjet printer sold by the Hewlett-Packard Company known as the THINK JET, the printhead comprises a heater plate and a fluid distributor plate. The heater plate is a glass substrate having the heating elements and addressing electrodes formed thereon with a hole drilled or isotropically etched, so that the ink can be fed through the heater plate to a shallow reservoir in the fluid distributor plate which is made by electroforming a material such as nickel over a three-dimensional mandrel. The apertures or nozzles in the fluid distributor plate are provided by thick film resist spot patterns formed on the mandrel prior to initiation of the electroform process. When the heater plate and the fluid distributor plate are aligned and bonded together, the contour of the fluid distributor plate forms the shallow reservoir mentioned above and the ink channels to the apertures that serve as droplet emitting nozzles. The ink travels through the drilled or etched hole and across the plane of the heater plate, thus also across the addressing electrodes, to the nozzles. There are two major disadvantages of this configuration. One is that it exposes the electrodes to the ink whenever there are any pin holes in the passivation layer. Secondly, the ink reservoir is quite shallow because it must be formed by the electroform. The shallow reservoir tends to permit the ink to dry out in the nozzles, causing first drop problems.
In the "roofshooter" printhead disclosed in U.S. Pat. No. 4,789,425 assigned to Xerox Corporation, the printhead comprises a silicon heater plate and a fluid directing structural member. The heater plate has a linear array of heating elements, associated addressing electrodes, and an elongated ink feed slot parallel with the heating element array. The structural member contains at least one recess cavity, a plurality of nozzles, and a plurality of parallel walls within the recess cavity which define individual ink channels for directing ink to the nozzles. The recess cavity and feed slot are in communication with each other and form the ink reservoir within the printhead. The ink holding capacity of the feed slot is larger than that of the recess cavity. The feed slot is precisely formed and positioned within the heater plate by anisotropic etching. The structural member may be fabricated either from two layers of photoresist, a two stage flat nickel electroform, or a single photoresist layer and a single stage flat nickel electroform.
The heater plate of the basic roofshooter-type thermal inkjet printhead can be modified to provide a four color printhead. When fabricating multi-colored printheads, the heater plate 28 (FIG. 1) must contain a feed slot 20 and an associated array of heating elements 34 for each color (usually black, magenta, cyan and yellow). When "passive resistor arrays" disclosed in U.S. Pat. No. 4,789,425 and shown in FIG. 1 are used, the electrical leads 33 for each resistive heating element 34 must run to the sides of the feed slot 20 and each resistive heating element 34 requires its own addressing electrode 32. The common return 35 for the heating elements also runs to the sides of the feed slot 20 and terminates at addressing electrodes 37. For multi-colors, it is desirable to place each color array on the same chip so that they are well aligned with one another. However, a problem arises in that each heater array consumes a large amount of surface area (referred to as silicon real estate) on the upper surface of each silicon wafer.
FIG. 2 shows one way of designing a four color roofshooter printhead using passive resistor arrays wherein the printhead is divided into two banks, each bank having two color feed slots (i.e., the first upper bank in FIG. 2 including black feed slot 20B and magenta feed slot 20M and the second lower bank including cyan feed slot 20C and yellow feed slot 20Y). While this design permits four color arrays to be placed on a single wafer subunit S, the printer is required to store information on two scan lines rather than one because of the two banks. While it would be desirable to place all four color arrays in a single bank, this is not practical because the inner color arrays consume considerable silicon real estate due to the fact that their electrical leads must all run to the sides.
U.S. Pat. No. 4,746,935 to Allen, assigned to Hewlett-Packard Company, discloses a method and apparatus useful for eight level halftone thermal inkjet printing by printing with droplets of ink having volumes weighted in a binary sequence. A four color roofshooter-type printhead which includes sets of three weighted drop generators for each color permits printing to be performed in eight levels with four colors.
U.S. Pat. No. 4,630,076 to Yoshimura discloses a four color ink jet printhead which additionally emits white or transparent ink droplets. This printhead includes multiple nozzles for each color. The structure for the present heater plate is not disclosed.
U.S. Pat. No. 4,549,191 to Fukuchi et al. discloses a multi nozzle ink drop-on-demand type of ink jet printing head which is able to deliver ink drops at a higher rate of speed through the use of capillary action. This printhead uses a driving transducer to form the droplets and does not disclose the multi-color printhead structure of the present invention.
U.S. Pat. No. 4,750,009 to Yoshimura discloses a multi-color ink jet printhead. This printhead includes a plurality of orifice groups (or nozzles) with each group being for a different color. One orifice group consists of a larger number of orifices than the other groups so that characters of higher definition can be printed out at a higher speed. The present invention is not taught or suggested by this reference.
There are also disadvantages to using a passive resistor array with a mono-color printhead. When a passive resistor array is used to address a plurality of heating elements 34, as shown in FIG. 1, the leads must be directed to the sides of feed slot 20. This creates a considerable gap "A" between the feed slot 20 and the end of chip 28. When two chips are butted to one another to form an array of chips (i.e., in forming a pagewidth printhead) a gap the size of two times "A" exists between adjacent feed slots 20. These gaps greatly reduce the resolution achievable since the number of nozzles per unit length is reduced.